This research project is concerned with the structure and properties of the hemoglobins of the sea cucumber Molpadia arenicola and how these respiratory pigments compare with those of other holothurians and with the hemoglobins of vertebrates. Structural studies in progress involve the determination of the sequence of a major M. arenicola globin chain utilizing tryptic, chymotryptic and cyanogen bromide cleavage. Physicochemical studies include an examination of the ligand binding characteristics of the M. arenicola hemoglobins by flash and steady state photochemical procedures and an investigation of the effects of pH, ionic strength and protein concentration on the oxygen equilibrium. M. arenicola hemoglobins undergo aggregation upon deoxygenation and this will be examined by gel filtration and ultracentrifugal methods. Further studies will be concerned with the immunological relatedness of the holothurian hemoglobins to each other and to the vertebrate hemoglobins.